The present invention relates to a tibial prosthesis which interacts with a femoral prosthesis to provide a knee joint replacement. The prosthesis is intended to be implanted in the proximal end of the tibia. The prosthesis of the present invention utilizes an intramedullary stem having a generally U-shaped cross section. The U-shaped cross section stem allows for retention of the cruciate ligaments of the knee while also increasing stability of the tibial prosthesis component.
The natural knee joint comprises the bottom (distal) end of the femur and the upper (proximal) end of the tibia. Bearing action occurs between two condyles on the lower surface of the femur and the complementary upper surface plateaus of the tibia, separated by intermediary cartilage pads, the meniscii. Connection of the tibia to the femur is provided by means of ligaments, including the cruciate ligaments of the knee, which are strong thick bundles situated between the condyles.
Movement of the normal knee is complex, including rolling, gliding and axial rotational motions. Not only do the cruciate ligaments provide connection between the femur and the tibia, assure joint stability and help to absorb stresses applied to the knee but they are also largely responsible for providing the proper combination of rolling and gliding motions and transitions between such motions which characterize the normal knee action.
Hip joint prosthetic devices are becoming relatively common and have had a fairly good record of success. Knee joint implants are of more recent design, have generally been less successful than hip implants and their long term stability has not yet been proven. The knee joint is subject to greater stresses than any other joint in the body. It must support the entire weight of the body above the knee and must do so throughout the various relative angular relationships of femur and tibia. Ideally, a knee joint prosthesis should provide the same action as the natural human knee--a complex combination of rotational, rolling and sliding/gliding movements.
Until recently, endoprosthetic knee joint devices comprised separate femoral and tibial components linked together with a coupling pin in the form of a mechanical hinge, having a single axis of rotation, fixed to the femur and the tibia, respectively, each component having a long intramedullary stem for bone fixation. These so-called hinge-type devices were not always duplicative of knee joint biological hingings since they had a single axis of rotation while the human knee joint which these devices sought to emulate involves polycentric movements. Thus, many variations of hinges, spindles, ball and socket and double hinges were used in an attempt to duplicate the complex natural knee motions; however, none of the simplified approximations could accurately do so. A common feature of these so-called hinged implants was that they provided positive mechanical connection between the femoral and tibial components so that the natural connective ligaments did not need to be and, in fact, could not be retained. Also, because of the bulk of these devices, it was necessary to remove a considerable amount of natural bone to allow space for the device, such removal reducing bone reserve which might be needed for future corrective measures.
A further disadvantage of the hinge-type devices was their limited axial rotation causing direct transmission of end limit forces through the structure thereby tending to loosen fixation of the femoral and tibial components. For this reason, practically all of these devices used all-metal, deep-bone penetrating intramedullary stems for purposes of fixation.
Examples of these hinge-type knee prosthetic devices can be found in Lagrange U.S. Pat. No. 3,688,316; Bousquet U.S Pat. No. 3,696,446; Goldberg U.S. Pat. No. 3,765,033; Findlay U.S. Pat. No. 3,886,601; Lagrange U.S. Pat. No. 3,918,101 and Arkangel U.S. Pat. No. 4,001,896.
The newer implantable knee devices generally mechanically uncouple the femoral and tibial components; instead of connecting the components by a pivot pin or other mechanical linkage they are held in mutual bearing engagement by the biological structure of the knee, that is, by indirect coupling through the muscular, capsular and ligamentous components of the natural joint. These devices usually comprise a combination of a femoral device constructed of metal with spaced runners to replace the natural condyles, and a plastic tibial device to replace the natural plateau, the devices having usually convex and concave mating surfaces, respectively.